Field of the Invention
This invention relates to electrostatic discharge (ESD) and improved ESD protection.
Description of the Related Art
FIG. 1 illustrates a portion of a radio frequency (RF) system that includes an integrated circuit 101 with an output stage 103 and a balun 105. Bond wires 107 couple the balun to an antenna 109. Baluns may be used to convert single-ended signals into differential signals or differential signals into single ended signals. Baluns may also be used to reject even harmonics and as band-select filters. In the example of FIG. 1, the balun comprises a transformer that includes an input coil 115 and an output coil 117. The number of turns in the illustrated RF system is the same in each coil although the ratio of turns in each coil can be different according to system requirements.
Integrated circuits with baluns, due to the isolation of the baluns, do not provide a robust electrostatic discharge to input/output terminals (pins) on chip. Conventional ESD protection circuits operate at frequencies comparable with normal balun operation and can interfere with balun operation. Thus, a conventional ESD protection circuit will respond to balun voltage/current swings as if it was an ESD event and will clamp to ground. That is undesirable since it will result in excessive leakage to ground during normal operation and possibly power loss.
The bond wires 107 include an inductance that provides impedance at high frequencies. Large voltage swings occur on terminals 108 and 110 as part of normal operation, e.g., a voltage swing of between −2 and +2 volts. The signals on terminals 108 and 110 are supplied to front-end module (FEM) 112, which in turn drives antenna 109. Typically, an FEM includes an input matching network to match the impedance to the driver power amplifier (PA), and/or includes another PA to get more power gain, and/or another low noise amplifier (LNA) to improve the noise figure of the received signal, and/or a matching network to match to the antenna 109.
Referring to FIG. 2, the positive output pad (Pad_outp) 201 supplies the positive signal to output terminal 203 through a bondwire 205 and the output pad (Pad_outn) 207 supplies the negative signal to output terminal 211 through bondwire 209. There is no built-in ESD path between the positive output (outp) 203 (or the negative output (outn) 211) and another input or output terminal on the integrated circuit shown as in/outx 215, which is coupled to integrated circuit 101 through bondwire 217 and PadX 219, and which is shown as an input/output terminal for generality. If outp (outn) is ESD stressed relative to in/outx, a large voltage will build between pins and an uncontrolled spark discharge 218 will occur as shown in FIG. 2 between outp and the relative ground 216 for the discharge event at in/outx. Since there is no safe place for the energy to be dissipated, the discharge will lead to damage to internal components. Because the balun has large positive and negative voltage swings on output pins during normal operation, an ESD protection network cannot be connected directly at the pins driven by the balun to provide protection. Thus, improved ESD protection is needed for baluns.